Numerous rapid solidification processes have been investigated for producing alloys with metastable phases and fine microstructures, such as, for example, fine dendritic structures. These processes are characterized by extremely rapid quenching and solidification of molten metal droplets or continuous streams as a result of contact of the molten alloy with a chill member or chill medium (e.g., a cooling gas). Such rapid solidification processes include splat quenching of molten droplets against one or more chill members, melt spinning of a continuous superheated molten stream onto chill rolls or wheels, powder making by melt atomization against a rotating disk and droplet solidification using a cooling gas curtain disposed around the disk, and plasma spray deposition of molten droplets onto a heat conductive substrate.
Drop tube processes have also been investigated for producing metastable phases and fine microstructures in metal alloy systems. In the drop tube process, a molten droplet is formed near the top of an evacuated elongate tube and released so as to fall by gravity through the drop tube. The molten droplet can be significantly undercooled (below its liquidus or freezing temperature) as it falls through the drop tube, solidifies and then is quenched in the quenching medium. The drop tube may be evacuated or may be backfilled with a cooling gas, such as helium, to provide convection cooling of the droplet in addition to radiation cooling.
As described in technical article "Metastable Structures In Drop Tube Processed Niobium Based Alloys", Adv. Space Res., Vol. 6, No. 5, pp. 123-126, 1986 investigators have combined deep undercooling of a molten droplet of Nb-Ge alloy during gravity fall through a drop tube with subsequent rapid solidification of the undercooled droplet in an attempt to minimize recalescence effects and provide metastable structures in the resulting solidified particulate. Rapid solidification of the undercooled droplet was effected by splat quenching the droplet on a stationary copper chill block at the bottom of the drop tube.
The drop tube processes described hereinabove are limited to production of small laboratory quantities (e.g., less than about 0.2 grams) of rapidly solidified particulate since the solidified particulate collects on the stationary copper chill block and must be removed therefrom after only one or a very small number of droplets are quenched thereon. Even such limited quantities of particulate are producible only on an intermittent, discontinuous basis as a result of the need to remove the particulate from the chill block. Moreover, the quench rate achievable in that process is limited by the stationary copper chill block. This limitation reduces the possibility of forming improved and/or unique metastable phases and structures in the undercooled/rapidly solidified particulate.